lustre/xattr: separate ACL and XATTR caches

This patch separates ACL and XATTR caches, so that
when updating an ACL only LOOKUP lock is needed and
when updating another XATTR only XATTR lock is needed.

This patch also reverts XATTR cache support for setxattr
because client performing REINT under even PR lock
can deadlock if an active server operation (like unlink)
attempts to cancel all locks, and setxattr has to wait
for it (MDC max-in-flight is 1).

This patch also drops mot_xattr_sem because there is
no use case for FLXATTRLS with locking.

This patch also takes into account that osd_xattr_list
may be changing lu_buf, so it does not assume that
the buf is unchanged after the call.

This patch disables the r/o cache if the data is
unreasonably large (larger than maximum single EA
size).

Signed-off-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-on: http://review.whamcloud.com/7208
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3669
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>